Direct Online Starter (DOL Motor Starter) : Circuit Diagram and Working Principle

Types of Starters for an Induction Motor

There are three types of starters mainly used for a squirrel cage induction motor.

• DOL( Direct Online Starter)

• Star Delta Starter

• Soft Starter

A DOL starter or Direct online starter is used for starting the small rating three phase induction motor.

Direct online or DOL Motor Starter:

The Direct On Line starter is the simplest form of motor starter. The direct online starter is a basic and simple starter used for starting the induction motor. In DOL Starter the stator winding of the induction motor is directly connected to  three phase supply voltage. Thus, the full line voltage is applied to motor terminal.

The DOL motor starter is suitable for starting small rating motors.  because the motor draws about 6 to 8 times of its rated current at starting time when  the full rated voltage is applied to stator terminals of the motor. The reasons of drawing high starting current are;

• The rotor is highly inductive because the rotor current frequency is slip times the stator frequency. The slip of the rotor current is unity at start. Thus, the rotor reactance is very high(X_L =2πsf_s). 

• At start, the induced voltage across the stator winding is zero and there is no opposing voltage to the applied voltage and the current limiting is through the stator winding resistance.

        I=(V-E)/R

        I =(V-0)/R

Because of above stated reasons, the motor draws about 6 to 8 times current of its full load current.

The power factor of the motor is very low and it draws large magnetizing current. The motor takes about 6-8 times when it is started and the magnitude of the current decreases as the motor gets accelerated towards its base speed. The starting torque of the motor is low at start.The DOL starting is suitable for the small rating induction motors.

If large rating induction motors are started with DOL starting method, a heavy starting current can damage the motor and also cause a disturbance in the power supply system because of the large dip in voltage.

The large rating induction motor can also be started on DOL if the capacity of power network permits. If the large rating transformer is used for distribution of three phases 440 volts, the large rating motor up to 90 KW can be started on DOL. However, in domestic connections the DOL starting of 5.5 KW motor is feasible.

The Direct on Line Starter usually consists of two parts:

1. Control Part      -  Contactors and Push Buttons
2. Protection Part   - Overload Relay

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Working Principle of DOL Starter :

The full line voltage is applied to stator for starting of three phase induction motor. The DOL starter is suitable for small rating three phase squirrel cage induction motor.

Direct Online Starter Circuit Diagram


The Direct online starter circuit diagram can be divided into two parts.

1. Power Circuit Diagram
2. Control Circuit Diagram

DOL Starter Power Circuit Diagram

The Power Circuit Diagram of the DOL starter consists following parts.

1. MCCB or Switch Fuse Unit
2. Motor Protection Circuit Breaker(MPCB) Or O/L Relay
3. Power Contactor of AC3 rating

The motor protection circuit breaker protects the motor under short circuit and the overloading condition.

DOL Starter Control Circuit Diagram

The control circuit of the DOL Starter is as given below.The control circuit has following components.

1. Auxiliary contactor
2. Start Push button
3. Stop Push Button
4. Indication Lamps- ON/OFF/Over Load

Working of DOL Starter :

When the start push button is pressed the contactor C2(main Contactor of AC3 rating ) operates and the full voltage is applied to the motor. The main contactor C2 remains in its on state through holding contact of the main contactor C2, after releasing the start push button. Thus the supply to stator is available after releasing the start button.

The motor draws large current and it starts reducing when the motor attains its base speed. The motor  current and torque is as given below.

When stop push button is pressed the main contactor C2 drops and motor stops.

In the case of jamming from the load side or problem in the motor like single phasing or unbalance voltage, the motor draws a large current above its full load current(FLC) and, the overload relay will get tripped with overloading fault. The contact of the overload relay QM1 drops the main contractor C2 and thus the motor stops due to overloading.The overload relay can be thermal over load type or electronic relay.

In electronic overload relay apart from the overload protection, the negative sequence current fault(I2) and earth fault protection(Io) and stall protection are also available and motor can be protected if fault occurs.

Advantages of DOL Starter

• Simple, most economical, easy to understand starter.

• More comfortable to design, operate and control.

• Provides nearly full starting torque at starting.

• Easy to troubleshoot.

• DOL starter directly connects the supply to the delta winding of the motor so there is no complexity of the  circuit diagram

Disadvantages of DOL Starter

• High starting current (5-8 times of full load current). The higher rating of switch gears are required to be installed.

• DOL Starter causes a significant dip in voltage hence suitable only for small motors. However, it can be used for starting a large rating motor up to 90 KW if the power transformer capacity permits. 

• DOL Starter reduces the lifespan of the machine because of higher thermal and mechanical stresses on the system.

Applications of DOL Starter

DOL starters are used to start the small rating motor like water pumps, compressors,conveyors etc. The compressor motor starts at no load therefore the DOL starting is suitable for compressor motor. The Squirrel cage induction motor coupled through fluid coupling can also be started with DOL starter.